possible cause shifting
Try inspecting the ratcheting mechanism located behind or under the clutch, inspect the oil draining out of the engine. is it clean or is there particulate in it? Look for any thing broken in the shift mech.
I was able to repair a Husaberg at the colorado gold rush 500 that had the same symptoms. One of the articulating pins located on the end of the shifting drum, a paul on the end of an arm or link interacts with these pins to do gear selection, had busted in half & was laying in the case right below were it belongs. No other damage had occurred.
Luckily, my friend Mr. Matt Sweeney ( a scca rally racer) had his support truck there & in it we found a drill bit that was the right size ! We ground it to the right length, installed it with loctite & staked it with a center punch to ensure it stayed put. The guys bike broke on day 2 of a 5 day ride, on day 3 we fixed it to the guys amazement of how simple the repair was. He was happy as a clam & finished the rest of the ride.
I was rewarded with free food & beer at dinner for the rest of the week. :ride:
IT WAS A FINE WEEK! Good Luck, Best regards, Porkchop ;)